In general, a vehicle wheel which is in contact with a road surface rotates according to a rotation of a steering wheel during traveling or stopping of a vehicle. In other words, when the steering wheel rotates to the left or right, the vehicle wheel rotates in the same direction as the rotation direction of the steering wheel. However, there may be a problem in that because the vehicle wheel is in contact with the road surface, the rotation amounts of the steering wheel and the vehicle wheel become different from each other due to a friction generated between the vehicle wheel and the road surface. For this reason, a driver needs a large force to manipulate the steering wheel.
The vehicle includes a power steering (PS) system as a steering force auxiliary device. In the power steering system, the coverage of an EPS scheme using an electric motor is being expanded in a passenger vehicle used in a real life.
For the purpose of power assistance, the power steering system is provided with a torque sensor that measures a deviation in the rotation angle between an input shaft side connected to the steering wheel and an output shaft side connected to the vehicle wheels in order to detect a torque load between the both shafts.
The torque sensor is largely divided into a contact type and a contactless type. Because the contact type entails a problem in that a noise is generated and durability is reduced, the contactless type has been preferred recently. In addition, the contactless type torque sensor is roughly classified into a magnetic resistance detection type, a magnetic deformation detection type, a capacitance detection type, and an optical detection type.
Meanwhile, a conventional magnetic resistance detection type torque sensor, which is provided in an electric power steering system, includes an input shaft whose upper end is coupled to the steering wheel manipulated by a driver and an output shaft whose upper end is coupled to a lower end of the input shaft through a torsion bar. A lower end of the output shaft is connected to a vehicle wheel. The lower end of the input shaft including the torsion bar and the upper end of the output shaft are covered by a housing, which has accommodated therein the torque sensor and the power means as described above. In this case, the input shaft includes a permanent magnet whose magnetic polarities are alternately arranged at regular intervals. Also, the output shaft is provided with a detection ring having a gear structure of which number of polarities correspond to the number of polarities of the permanent magnet and which is made of a ferromagnetic substance that can generate a magnetic induction caused by the permanent magnet included in the input shaft. The detection ring is constructed such that a sensor for detecting magnetism is connected thereto. In this case, a relative twist between the permanent magnet provided at the input shaft and the detection ring of the gear structure provided at the output shaft causes a change in area where the permanent magnet and the detection ring face each other. Accordingly, a magnetic flux is changed in the detection ring and the change of the magnetic flux is detected by the sensor so that a twist angle of the output shaft relative to the input shaft can be detected.
However, the conventional contactless type torque sensor encounters problems in that an excessive number of constituent elements are required and an assembly process is complicated, thus leading to increases in the possibility of erroneous operation and the manufacturing cost, and to an exposure of a problem associated with durability period of the torque sensor due to the excessive number of constituent elements.